The present invention relates to a brake power booster for automotive vehicles including a booster housing having its interior subdivided by a movable wall into a first chamber (vacuum chamber) and a second chamber (working chamber), and a control valve which controls a pneumatic pressure differential that acts upon the movable wall, the control valve having at least two sealing seats, which interact with an elastic valve member, and being operable by an actuating rod, on the one hand, and by a solenoid irrespective of the actuating rod, on the other hand, the armature of the solenoid being in force-transmitting cooperation with one of the sealing seats.
A brake power booster of this type is disclosed in international patent application No. WO 94/11226, for example. A disadvantage in the prior art brake power booster is the drop of forces in the force-travel characteristic curve of the control valve which occurs especially during the inflow of outside air into the working chamber and causes instabilities. To adapt the course of the force characteristic curve of the solenoid to the mentioned consumer characteristic curve, a highly dynamic and precise, complicated control system is required which needs data about the actuating travel of a sleeve. The sleeve carries the sealing seat operable by the solenoid. A sophisticated sensor structure is required to this end.
Therefore, an object of the present invention is to disclose measures which permit eliminating the need for the sensor structure and simplifying the control system in addition. More particularly, the objective is to position unstable points on the consumer characteristic curve.